Abstract [en]

The stability of natural bentonite suspensions has been investigated as a function of temperature at pH 9 and ionic strength 10-3 M. The sedimentation rate of the particles is directly related to their stability. The sedimentation kinetics was determined by examining the variation of particle concentration in solution with time. The observed kinetics for sedimentation is discussed quantitatively in terms of the potential energy between particles. The ζ-potential of the particles was measured and the DLVO theory was used to calculate attractive and repulsive potentials. Experimental observations are consistent with DLVO model predictions and show that the stability of bentonite colloids increases with temperature. Differences with other colloidal systems can be attributed to the temperature dependence of the surface charge of bentonite particles.

Abstract [en]

In deep geological repositories in Sweden, encapsulated nuclear waste will be surrounded by compacted bentonite in the host rock. In future contact with water-bearing fractures, this bentonite barrier can release montmorillonite colloids under certain conditions. This process can lead to loss of buffer material. Furthermore, these colloids, if stable, may facilitate the transport of associated radionuclides towards the biosphere. Colloid stability is determined by groundwater chemistry.

This study addresses the effects of groundwater chemistry on the stability of montmorillonite colloids. During the lifetime of the repository, the pH and ionic strength of the groundwater are expected to vary, partly due to intrusion of glacial melt water. Initially, the temperature will be higher in the surrounding host rock due to heat released from radioactive decay in the spent nuclear fuel. The effects of these parameters on the stability of montmorillonite suspensions were evaluated by studying the aggregation kinetics. The change in particle concentration with time was monitored by Photon Correlation Spectroscopy (PCS).

Aggregation kinetics experiments showed that for a given pH and temperature, the rate constant for colloid aggregation increased with increasing ionic strength. The relationship between the rate constant and the ionic strength allowed the NaCl and CaCl2 critical coagulation concentration (CCC) for Na- and Ca-montmorillonite to be determined.

The aggregation rate constant decreased with increasing pH as the surface potential increased. This effect became more pronounced at higher ionic strengths and higher temperatures but could not be observed at low temperature.

The effect of temperature on the stability of the suspensions is pH-dependent. At pH≤4, the rate constant for colloid aggregation increased with increasing temperature, regardless of ionic strength. At pH≥10, the aggregation rate constant decreased with increasing temperature. In the intermediate pH interval, the aggregation rate constant decreased with increasing temperature except at the highest ionic strength, where it increased.

García García, Sandra

KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.

2010 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

In Sweden the encapsulated nuclear waste will be surrounded by compacted bentonite in the granitic host rock. In contact with water-bearing fractures the bentonite barrier may release montmorillonite colloids that may be further transported in groundwater. If large amounts of material are eroded from the barrier, the buffer functionality can be compromised. Furthermore, in the scenario of a leaking canister, strongly sorbing radionuclides, can be transported by montmorillonite colloids towards the biosphere. This thesis addresses the effects of groundwater chemistry on the generation, stability, sorption and transport of montmorillonite colloids in water bearing rock fractures.

To be able to predict quantities of montmorillonite colloids released from the bentonite barrier in contact with groundwater of varying salinity, generation and sedimentation test were performed. The aim is first to gain understanding on the processes involved in colloid generation from the bentonite barrier. Secondly it is to test if concentration gradients of montmorillonite colloids outside the barrier determined by simple sedimentation experiments are comparable to generation tests. Identical final concentrations and colloid size distributions were achieved in both types of tests.

Colloid stability is strongly correlated to the groundwater chemistry. The impact of pH, ionic strength and temperature was studied. Aggregation kinetics experiments revealed that for colloid aggregation rate increased with increasing ionic strength. The aggregation rate decreased with increasing pH. The temperature effect on montmorillonite colloid stability is pH-dependent. At pH≤4, the rate constant for colloid aggregation increased with increasing temperature, regardless of ionic strength. At pH≥10, the aggregation rate constant decreased with increasing temperature. In the intermediate pH interval, the aggregation rate constant decreased with increasing temperature except at the highest ionic strength, where it increased. The relationship between the rate constant and the ionic strength allowed the critical coagulation concentration (CCC) for Na- and Ca-montmorillonite to be determined.

In order to distinguish the contribution of physical filtration and sorption to colloid retention in transport, the different retention mechanisms were quantified. Sorption on different representative minerals in granite fractures was measured for latex colloids (50, 100, 200 nm) and montmorillonite colloids as a function of ionic strength and pH. Despite of the negative charge in mineral surfaces and colloids, sorption was detected. The sorption is correlated to the mineral point of zero charge and the zeta potential of the colloids, and increases with increasing ionic strength and decreasing pH. In transport experiments with latex colloids in columns packed with fracture filling material, the retention by sorption could clearly be seen. In particular at low flow rates, when the contact time for colloids with the mineral surfaces were the longest, sorption contributed to retention of the transport significantly. The retention of latex colloids appeared to be irreversible in contrary to the reversible montmorillonite colloid retention.

Generation, stability and sorption of the montmorillonite colloids are controlled by electrostatic forces; hence, the results were in qualitative agreement with DLVO.